1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an optical sheet and a backlight unit using the same in which deviation of a viewing angle is minimized to improve picture quality.
2. Discussion of the Related Art
Recently, various flat panel displays have been developed with reduced weight and volume as compared to a cathode ray tube device. Examples of the flat panel displays include liquid crystal display (LCD) devices, field emission display (FED) devices, plasma display panel (PDP) devices, and light emitting display (LED) devices.
The LCD device displays desired images by controlling light from a backlight unit using an LCD panel. The LCD panel includes a plurality of liquid crystal cells and a plurality of control switches for switching video signals supplied to the respective liquid crystal cells. The backlight unit includes prism and diffusion sheets that can efficiently condense and diffuse light.
Hereinafter, related art prism and diffusion sheets will be described with reference to the accompanying drawings. FIG. 1 illustrates the related art prism and diffusion sheets.
As shown in FIG. 1, a related art prism sheet 11 includes a condensing sheet 12, and a plurality of prism peaks 13 formed on the condensing sheet 12. Each of the prism peaks 13 has a triangular prism shape and their apexes are arranged on the condensing sheet 12 opposing a diffusion sheet 10. The prism peaks 13 condense light emitted from a lamp and transmit the condensed light to the diffusion sheet 10.
The prism sheet 11 more efficiently condenses either light diffused in an X-axis direction or a Y-axis direction along an arrangement direction of the prism peaks 13. Specifically, the prism sheet 11 efficiently condenses light diffused in a tilt direction of the prism peaks 13, i.e., in a Y-axis direction of FIG. 1. By contrast, the prism sheet 11 fails to efficiently condense light diffused in a longitudinal direction of the prism peaks 13, i.e., an X-axis direction of FIG. 1. Accordingly, the light which has passed through the prism sheet 11 is less diffused in the Y-axis direction because it is well condensed. However, the light is much more diffused in the X-axis direction because it is not well condensed.
Afterwards, the light which has passed through the prism sheet 11 passes through the diffusion sheet 10 and is then irradiated toward a rear surface of an LCD panel. At this time, since the condensing efficiency depends on each direction, the light irradiated toward the rear surface of the LCD panel causes the following problems.
FIG. 2 illustrates viewing angle characteristics depending on the condensing efficiency. As described above, since the light is not well condensed in the X-axis direction but is more efficiently condensed in the Y-axis direction, a viewing angle θ1 in the X-axis direction from the rear surface of the LCD panel is wide but a viewing angle θ2 in the Y-axis direction is narrow. A problem occurs in that picture quality is degraded due to deviation between the viewing angle θ1 in the X-axis direction and the viewing angle θ2 in the Y-axis direction. Generally, the viewing angle θ1 in the X-axis direction is about 45° while the viewing angle θ2 is about 32°.